1. Field of the Invention
The present invention relates to an exposure amount control device suitably applied to a slit scan type exposure apparatus for controlling an exposure amount and uniformity of luminous intensity to a photosensitive substrate within a predetermined range wherein a pattern of a mask is exposed on the photosensitive substrate by synchronously scanning the mask and the photosensitive substrate with respect to, e.g., a rectangular or circular illumination area illuminated with a pulsed light source as an exposure light source.
2. Related Background Art
In manufacturing semiconductors, liquid crystal display devices or thin film magnetic heads, etc. under photolithography technique, projection exposure apparatuses have been utilized in which the pattern of a photomask or a reticle (hereinafter called as "the reticle") is exposed via a projection optical system on a photosensitive substrate such as a wafer with photoresist applied thereto, a glass plate or the like. Recently, a chip pattern of a semiconductor, etc. tends to become large and in the projection exposure apparatus, it is required to expose a larger portion of the pattern of a reticle on the photosensitive substrate.
Also, as the pattern of a semiconductor, etc. becomes minute, improvement of resolution of the projection optical system is required. In order to improve the resolution of the projection optical system, the exposure field of the projection optical system needs to be enlarged, which is difficult in respect to the design or manufacture. Especially, when using a reflective and refractive system as the projection optical system, there is a case that the shape of the exposure field with no aberration happens to be circular.
In order to deal with the problems of the tendency of enlargement of the pattern to be exposed and the limitation of the exposure field of the projection optical system, a so-called slit scan type projection exposure apparatus has been developed in which the pattern of a reticle larger than a slit-shaped illumination area on the reticle is exposed on a photosensitive substrate by synchronously scanning the reticle and the photosensitive substrate with respect to, e.g., a rectangular, circular or hexagonal illumination area (hereinafter called as "the slit-shaped illumination area"). Generally, in the projection optical apparatus, the condition of a proper exposure amount and uniformity of luminous intensity with respect to a photosensitive material of the photosensitive substrate is determined. Therefore, the slit scan type projection exposure apparatus is also provided with an exposure amount control device which makes an exposure amount to the photosensitive substrate coincide with a proper exposure amount within a predetermined allowable range and maintains the uniformity of the luminous intensity of exposure light to the wafer within a predetermined level.
Also, recently, it is required to enhance the resolution of the pattern to be exposed on the photosensitive substrate. A method for enhancing the resolution is to use shortwave exposure light. Among presently usable light sources, an excimer laser light source, a pulsed oscillation type laser source (pulsed light source) such as a metallic vaporization laser light source or the like emits shortwave exposure lights. However, the exposure energies (light amounts) of the pulsed light emitted from the pulsed light source, different from the continuous light emission type light source such as a mercury lamp or the like, are fluctuated for the respective pulses within a predetermined range.
Consequently, in the conventional exposure amount control device, when the average pulsed light amount of pulsed light emitted from a pulsed light source on the photosensitive substrate is &lt;p&gt; and the range of the fluctuations of the light amounts of the pulsed light is .DELTA.p, the parameter .DELTA.p/&lt;p&gt; representing the fluctuations of the light amounts of the pulsed light is deemed to become a normal distribution (actually random). When the number of light pulses emitted to a certain area including a predetermined width in the scan direction (pulsed light integrated area) on the photosensitive substrate which is scanned relatively with respect to an exposure area conjugate to the slit-shaped illumination area is N, the exposure amount control device is controlled such that the integrated exposure amount reaches a proper exposure amount within a predetermined allowable range by using the fact that the fluctuations of the integrated exposure amount after the end of exposure becomes (.DELTA.p/&lt;p&gt;)/N.sup.1/2.
Also, as disclosed in U.S. Pat. No. 4,822,975, when performing exposure by a method in which positions of the photosensitive substrate and the reticle are measured and the light emission trigger is sent to the pulsed light source synchronously with the result of the measurement, fluctuations of light emitting time (positions) may occur when, after performance of actual measurement in the measurement unit (laser interferometer or the like), the result of the measurement is outputted as a factor of the exposure apparatus.
Such fluctuations of light emitting positions can occur both in a large area and in a small area of the integrated exposure amount locally on the photosensitive substrate after scan and exposure. Because if light intensity on the slit-shaped illumination area arises from 0 to 100% in a step function on the adjacent non-illumination area, the width of the slit-shaped exposure area on the photosensitive substrate in the scan direction is represented by D and pulsed light emission is performed each time when the photosensitive substrate moves by D in the scan direction (which corresponds to the case that the number N of pulses illuminated on the pulse number integrated area is 1), the integrated exposure amount on joint portions between the adjacent pulse number integrated area can become double or 0 depending on positioning accuracy of the photosensitive substrate.
Thus, regarding the case that pictures (chip patterns) are joined in the non-scan direction by using a light source continuously emitting such as a mercury lamp or the like (which corresponds to the case that the number N of pulses using the pulsed light source in the scan direction is 1), there is disclosed a method in U.S. Pat. No. 3,538,828 in which the light intensity distribution in the non-scah direction (i.e., the direction of joining pictures), which integrates the slit-shaped exposure area in the scan direction, is formed to be an equal-legs stand shape. This method is adopted to the case that the number N of pulses illuminated at each point on the photosensitive substrate is 1 so that the inconvenience of which the integrated exposure amount fluctuates on the joint can be reduced. Although it is actually apparent from the disclosure of the U.S. Pat. No. 3,538,828, that there is also disclosed another method in U.S. Pat. No. 4,822,975 such that the luminous intensity distribution in the direction of joining pictures (in the scan direction) is formed to be an isosceles triangle or an equal-legs stand shape.
In the prior art described above, it is possible qualitatively to correspond to only when the value of the width of the light intensity on the slit-shaped illumination area in the scan direction and the distance of the light emitting position are equal to each other (when the number N of pulses on the pulse number integrated area is 1), a condition for obtaining a desired exposure amount and the uniformity of luminous intensity has not been revealed. Also, when performing exposure such that the number N of pulses is more than 2 and a plurality of pulses are overlapped on each pulse number integrated area of the photosensitive substrate, the condition for obtaining a desired exposure amount and the uniformity of luminous intensity has not been clear.